Presbyopia is a state where an accommodation function by a crystalline lens of an eye is deteriorated and near vision is difficult. A progressive power lens is used for glasses for correcting presbyopia.
In general, the progressive power lens is an aspherical lens provided with two refracting areas with different refracting powers and a progressive area between the both areas where a refracting power (dioptric power) is progressively changed, which enables far vision and near vision with a single lens without a boundary. The two areas refer to a far-sighted part region set at an upper part of a lens and a near-sighted part region set at a lower part of the lens. A progressive region, which is a transfer zone between the far-sighted part region and the near-sighted part region are connected smoothly and continuously.
The far-sighted part region is mainly a region for viewing an object at a far distance, the near-sighted part region is mainly a region for viewing an object at a near distance, and the progressive region is mainly a region for viewing an object at an intermediate distance. However, the refracting power is continuously changing in the progressive power lens, and these areas are not clearly divided.
Since the progressive power lens has asymmetrical distribution of optical performance, its image distortion is more remarkable than that of a spherical lens. Also, the shallower a base curve of a lens becomes, the thinner the progressive power lens can be formed, but distortion aberration and astigmatism tend to become larger. Thus, how to reduce the distortion aberration and astigmatism has been a problem particularly in a thin-type progressive power lens.
Therefore, a progressive power lens for improving the distortion aberration has been proposed. Patent Document 1 is cited as an example of such a progressive power lens.    Patent Document 1: Japanese Patent Laid-Open No. 2004-264365